我有一个Haskell类型类问题 . 我不能通过语法来获得这个(看似合理的)程序来在GHC下编译 .
import Control.Concurrent.MVar
blah1 :: [a] -> IO ([a])
blah1 = return
blah2 :: [a] -> IO (MVar [a])
blah2 = newMVar
class Blah b where
blah :: [a] -> IO (b a)
instance Blah [] where
blah = blah1
-- BOOM
instance Blah (MVar []) where
blah = blah2
main :: IO ()
main = do
putStrLn "Ok"
我收到以下错误消息,哪种有意义,但我不知道如何解决它:
`[]' is not applied to enough type arguments
Expected kind `*', but `[]' has kind `* -> *'
In the type `MVar []'
In the instance declaration for `Blah (MVar [])'
2 回答
你想要的不是直接表达的 . 这可能就像你得到的那样接近:
我正在阅读关于Conal Elliott的TypeCompose图书馆的文章,并提醒了这个问题 . 这是一个如何进行类型级组合的示例 .